As @joshuad11 already asked - are you using the same cable?
The most common cause of issues are the cables, the bending of them causes small fractures in the copper core and that slowly deteriorates the cable, the longer the cable the more places this can occur.
Are you using the SAME cable between the phone and the Powercore 20100 as the same cable between the phone and the Powercore 10000?
If this is the SAME cable then it probably means your 20100 has an issue.
If it is a DIFFERENT cable then consider swapping cables over and see if the problem moves with the cable.